Considering the rising prices of crude oil, the refiners are forced to process opportunity crude such as DOBA, to be competitive. However these opportunity crudes pose many problems such as fouling of heat exchangers, difficulties in effluent treatment, poisoning of catalyst by certain metallic salts and such other problems.
Among the metals, calcium poses very serious problems which cannot be tackled using the current refinery processes. Calcium exists in crude oil as calcium complex of naphthenic acid, which hereinafter is referred to as calcium naphthenate. The calcium naphthenate is not removed from the crude oil during the normal desalting process. The examples of the type of crude oil which contains large amounts of calcium naphthenate are crudes from China such as Shengli No. 2; DOBA from West Africa; Gryphon and Harding crude oil from the North Sea; and SJV from the West Coast of USA.
In an oil refinery, the desalting of crude oil has been practiced for many years. The crude is usually contaminated from several sources, including, metals including calcium, zinc, silicon, nickel, sodium, potassium, and such other metals.
Desalting is necessary prior to further processing to remove these compounds and other inorganic materials that would otherwise cause fouling and deposits in downstream heat exchanger equipment and/or form corrosive salts detrimental to crude oil processing equipment. Further, these metals can act as poisons for the catalysts used in downstream refinery units. Effective crude oil desalting can help minimize the effects of these contaminants on the crude unit and downstream operations. Proper desalter operations provide the following benefits to the refiner:
(a) Reduced crude unit corrosion.
(b) Reduced crude preheat system fouling.
(c) Reduced potential for distillation column damage.
(d) Reduced energy costs.
(e) Reduced downstream process and product contamination.
Desalting is the resolution of the natural emulsion of water that accompanies the crude oil by creating another emulsion in which about 5 percent relative wash water is dispersed into the oil using a mix valve. The emulsion mix is directed into a desalter vessel containing a parallel series of electrically charged plates. Under this arrangement, the oil and water emulsion is exposed to the applied electrical field. An induced dipole is formed on each water droplet within the emulsion that causes electrostatic attraction and coalescence of the water droplets into larger and larger droplets. Eventually, the emulsion resolves into two separate phases—the oil phase (top layer) and the water phase (bottom layer). The streams of desalted crude oil and effluent water are separately discharged from the desalter.
The entire desalting process is a continuous flow procedure as opposed to a batch process. Normally, chemical additives are injected before the mix valve to help resolve the oil/water emulsion in addition to the use of electrostatic coalescence. These additives effectively allow small water droplets to more easily coalesce by lowering the oil/water interfacial tension.
Crude oil that contains a high percent of particulate solids can complicate the desalting process. The particulate solids, by nature, would prefer to transfer to the water phase. However, much of the solids in a crude oil from a field exist in tight water-in-oil emulsions. That is, oil-wetted solids in high concentration in the crude may help form tight oil and water emulsions that are difficult to resolve. These tight emulsions are often referred to as “rag” and may exist as a layer between the separated oil and water phases. The rag layer inside the desalter vessel may grow to such an extent that some of it will be inadvertently discharged with the water phase. This is a problem for the waste water treatment plant since the rag layer still contains a high percentage of unresolved emulsified oil.
Much of the solids encountered during crude oil desalting consists commonly as particulates such as iron oxide, iron sulfide, sand, clay and even phosphorus-containing compounds, etc. Other metals that are desirably removed include, but are not necessarily limited to, calcium, zinc, silicon, nickel, sodium, potassium, and the like, and typically a number of these metals are present. Some of the materials may be present in a soluble form, and some may require modification through reaction such as reaction or neutralization to become soluble. The metals may be present in inorganic or organic forms. In addition to complicating the desalter operation, phosphorus and other contaminants are of particular concern to further downstream processing. This includes the coking operation since iron and other metals remaining in the processed hydrocarbon yields a lower grade of coke. Removing the metals from the crude oil early in the hydrocarbon processing stages is desired to eventually yield high quality coke as well as to limit corrosion and fouling processing problems.
Several treatment approaches have been made to reduce total contaminant levels and these all center on the removal of contaminants at the desalter unit. Normally, the desalter only removes water soluble inorganic salts such as sodium or potassium chlorides.
Basic metals such as calcium, when present in crude oil can lead to fouling of heaters and heat exchangers and poison catalysts used in crude processing. When present as inorganic salts, such as, chlorides, usually in oil-encapsulated water phase, the salts can hydrolyze to release corrosive mineral acids. Refinery desalters customarily remove such salts. However, oil-soluble metal salts such as naphthenates and phenolates are not removed by conventional desalting. Therefore, oil-soluble, basic metal-rich crudes are less valuable than crudes with low levels of such metals. A process for metal ion removal enables the increase of the value of such crudes.
A few, but increasingly important, petroleum crude feedstocks, residua, and deasphalted oil derived from them, contain levels of calcium or iron which render them difficult, if not impossible, to process using conventional refining techniques. The metals contaminants causing particular problems are in the form of nonporphyrin, organometallically bound compounds. These species have been attributed to either naturally occurring calcium complexes or solubilized calcium from recovery waters that comes in contact with crude oils. One possible class of calcium compounds identified in particular is the respective naphthenates and their homologous series. These organometallic compounds are not separated from the feedstock by normal desalting processes, and in a conventional refining technique they can cause the very rapid deactivation of hydroprocessing catalysts. Examples of feedstocks demonstrating objectionably high levels of calcium compounds are crudes from China such as Shengli No. 2; DOBA from West Africa; Gryphon and Harding crude oil from the North Sea; and SJV from the West Coast of USA.
US Patent Application 20050241996 describes the use of only poly (acrylic acid) derivatives, (that is, polymers) for removing metal irons from hydrocarbon feedstocks. Even though this patent has listed 16 representative non-ionic water soluble monomers, 27 representative anionic monomers and 30 cationic monomers, wherein list of anionic monomers include maleic acid and fumaric acid, there is absolutely no suggestion or teaching in this patent, that any of these monomers can be used independently or in combination for removing metal ions from the hydrocarbon feedstocks. There is insistence in this patent on use of aqueous solution of only one or more water-soluble poly (acrylic acid) derivatives, that is use of polymers for the purpose of this US Patent Application.
It is known to a person skilled in the art that, it is necessary that a catalyst is used to react with a monomer of an acid to form its derivatives in a polymeric form. This adds to the cost of the process due to time involved and equipments and chemicals used in the process and such other factors.
In addition, it is observed by the inventor of present invention that when poly (acrylic acid) derivative of US Patent Application 20050241996 is used, (that is, ACUMER-1000 is used), heavy precipitation takes place, which can lead to fouling of the processing equipments. This is clear from the data provided in Table 6, Experiment No. 1 of the present specification. Also to prevent this precipitation higher dosages of the additive are required. The higher dosage will lead to higher cost. Other disadvantage of using additives having a tendency to precipitate is that it will be difficult to control the dosage at the desired level in the equipments in the field, such as crude desalter, and hence additive will have to be used always in excess.
US Patent Application 2005/0241997 A1 describes different additives useful for enhancing phosphorous compound removal in refinery desalting process. Reactive phosphorus species can be removed or transferred from a hydrocarbon phase to a water phase in an emulsion breaking process by using a composition that contains water-soluble hydroxy acids. Suitable water-soluble hydroxy acids include, but are not necessarily limited to glycolic acid, gluconic acid, C2-C4 alpha-hydroxy acids, poly hydroxy carboxylic acids, thioglycolic acid, chloro acetic acid, polymeric forms of the above hydroxyacids, poly-glycolic esters, glycolate ethers, and ammonium salt and alkali metal salts of these hydroxyacids, and mixtures thereof. The composition may optionally include a mineral acid to reduce the pH of the desalter wash water. A solvent may be optionally included in the composition. This US Patent Application permits transfer of reactive phosphorus species into the aqueous phase with little or no hydrocarbon phase undercarry into the aqueous phase. The composition is particularly useful in treating crude oil emulsions, and in removing calcium and other metals therefrom.
This US Patent Application 2005/0241997 A1, teaches the use of only hydroxyl mono-carboxylic acids such as, glycolic acid and polyhydroxy derivative thereof, like gluconic acid as an additive compound for removal of reactive phosphorous species, and calcium and other metals, from the hydrocarbon feedstock. However, the disadvantage of the use of these acids and derivatives as additives compound, as seen from the experiments conducted by the present inventor to remove calcium from calcium napthenate from hydrocarbon feedstock, is that these acids require higher dosages as additive compound since they are to be used in 2:1 molar ratio with respect to calcium. When gluconic acid was used as additive compound by the present inventor, in the same molar ratio, that is, 2:1, very high dosage of gluconic acid is required.
The inventor of the present invention, after extensive experimentation, has surprisingly found that the use of any of the esters of various dicarboxylic acids such as monomethyl maleate, monomethyl oxylate, dimethyl maleate and ester of tricarboxylic acids such as citric acid and also esters of polycarboxylic acids is very effective in removal of metals like calcium and iron from hydrocarbon feedstock. The prior art has never mentioned use of above mentioned esters for this purpose. It is surprisingly found by the present inventor that among all the esters of carboxylic acids, only a few do not lead to precipitation of calcium salt. For example, ester of maleic acid does not lead to any precipitation.
Thus it will be seen that the prior art mentions that the use of carboxylic acids is effective in removal of calcium from the hydrocarbon feedstock. However, the inventor of the present invention has surprisingly found that the use of esters of carboxylic acids is very effective in removal of calcium from the hydrocarbon feed stock.
In view of above, there is a need for developing a new method for the effective removal of metal contaminants, particularly calcium, from hydrocarbon feedstocks, including crude oil.